This tag is for questions about the exact nature of wavefunction collapse.

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How does linearity of a measurement imply that the commutator of all measured observables are $c$-numbers?

I really don't understand with the linearity conditions I have where this comes from.
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582 views

Does an electron move from one excitation state to another, or jump?

I'm wondering, when an electron changes state, does it move from one state to another over some (very small) time period? Or does it change from one state to another in no time? If the former, what ...
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1answer
222 views

What happens to entangled particles when momentum is measured?

In Wikipedia it is mentioned that position and momentum can be entangled as well as spin and polarization etc. I assume etc. is charge etc. I understand how if you measure spin up on one of a pair you ...
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1answer
103 views

Weird Behaviour of the act of measurement to a quantum system

I and my friend were disputing about some weird behaviour of the act of measuring some observables quantities e.g. Energy, position. But I still don't think what he said is strictly true. He said" ...
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2answers
140 views

How does interaction of a system with the environment lead to the damping of interference terms?

A general way to describe a system $S$ that is entangled with an environment $E$ is $\rho_{S}=Tr(\rho_{SE})=\sum\limits_{m,n}c_mc^*_n |s_m\rangle \langle s_n| \langle e_n|e_m\rangle$ with ...
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194 views

More Heisenberg Uncertainty Principle (HUP) Clarification

If you look at the commutation relation of the position and momentum operators (in 1D position space), you get: $$[\hat{x}, \hat{p}_x] = [x,-i \hbar \frac{\partial}{\partial x}] = i \hbar$$ All this ...
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0answers
101 views

Collision of 2 neutrons

If two neutrons collide in 3D space and we want to determine the final velocities of both nuetrons (3 components for each neutrons), we can use the conservation of momentum equations and the ...
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1answer
312 views

Why doesn't gravity act as a measurement?

I think this must be a very basic question but I couldn't find the answers anywhere. I was starting reading about Quantum Mechanics and these questions came in mind: As I understand the quantum ...
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2answers
631 views

Quantum mechanics potential barrier problem [duplicate]

While reviewing some quantum mechanics, I cam across a very interesting situation. For a potential barrier, if a particle has an energy $E$ less than the potential barrier $V_0$, it is possible to ...
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2answers
2k views

Expectation Values in Quantum Mechanics

Why is the expectation value what it is? Why don't you apply the operator, then multiply that by it's conjugate?
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1answer
1k views

A Simple Explanation for the Schrödinger Equation and Model of Atom? [closed]

I tried reading the Wikipedia article to no avail - I simply cannot understand the Schrödinger Equation (what does each of the variables mean, especially the wave function), and the Schrödinger Model ...
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1answer
148 views

Can an AC magnetic field be measured by a hall probe?

Many hall sensors on the market have a datasheet that explain the proportional relationship between the analog voltage that is outputted and the relative gauss/tesla reading (for example: 2.5mV = ...
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2answers
159 views

Non-locality and Bell's theory

Non-Locality – (just ) one more question? I have read comments that Bell’s theory proves quantum mechanics is non-local, and also comments that it does not. I have read a comment by a very eminent ...
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3answers
319 views

Confusion about wavefunction separability

A wavefunction is inherently a multi-particle function. If you have a container that is perfectly isolated from the external universe (not possible, but just imagine it) and filled with $n$ ...
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1answer
323 views

If there is no collapse of the wave-function does this mean that the many worlds interpretation of QM must be wrong?

If as some people suggest, there is no collapse of the wave function (is there a standard name for this position), then must one rule out the many-worlds interpretation of QM?
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2answers
574 views

What happens after the collapse of a wavefunction?

If I have a quantum system which I prepare in a certain state, this state then evolves unitarily via a Hamiltonian. Suppose an observer provokes a collapse of the wave function by a certain ...
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1answer
251 views

Can observers be particles?

Generally Quantum mechanics divides a system what is to be observed and an observer. This is generally taken to be some human being. But why restrict it to such? Why not a particle? Is there a good ...
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4answers
1k views

Scientist propose solution to quantum measurement problem, explain further

Link to Article The scientists In their recent opus magnum, Theo Nieuwenhuizen (Institute of Physics, UvA) and colleagues claim to have found a solution to the so-called quantum measurement ...
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Eigenstate of position+momentum?

I'm studying Quantum Mechanics on my own, so I'm bound to have alot of wrong ideas - please be forgiving! Recently, I was thinking about the quantum mechanical assertion (postulate?) that states with ...
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1answer
140 views

A question about quantum measurement and associating a linear self adjoint operator to it

I have a question about the concept of measurement and observable in quantum mechanics. I'd like to fist explain my understanding of it and then ask the question. First we have a system and its ...
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3answers
171 views

Measurements and simultaneity [duplicate]

Picture a situation where we have two observers, $A$ and $B$, and a system in a certain quantum state. If $B$ makes a measurement of some observable, say energy for example, the state will collapse to ...
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2answers
413 views

How do you come up with a POVM?

This is a made-up example, just to understand a concept. If changing the probability values aids your explanation, that's fine by me. Say you have a physical quantity $E$ that can take values 1, 2, 3 ...
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1answer
73 views

Is it possible to determine the state or values of something without measuring it

To give context to this question, I am currently looking into non-locality / hidden variables / Bell's Theorem, EPR / etc. I've noticed the assertion that the values / state of something when ...
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2answers
312 views

Can we measure “wavefunction” of quantum particles?

We know that there is uncertainty principle, so question: can we ever measure wavefunction of particles? I do not think this is possible, but I am not sure. I guess that everything is probabilistic. ...
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4answers
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Is every quantum measurement reducible to measurements of position and time?

I am currently studying Path Integrals and was unable to resolve the following problem. In the famous book Quantum Mechanics and Path Integrals, written by Feynman and Hibbs, it says (at the beginning ...
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Does quantum mechanics depend solely on electromagnetic waves? [duplicate]

I am beginning to learn quantum mechanics. Since determining the position of an object involves probing by electromagnetic waves and since i have read a simple derivation of Heisenberg's uncertainty ...
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1answer
566 views

How do we know superposition exists?

How do we know superposition exists? Has it been observed, or has it been deduced, and how certain are we? The Copenhagen Interpretation seems to imply that superposition collapses into one state ...
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5answers
318 views

How do we know that there isn't a classical solution to the measurement problem/Quantum Mechanical uncertainty?

It was mentioned to me that it can be shown that there is no classical explanation for the uncertainty in Quantum Mechanics -- i.e. that there are no hidden workings that we have just not yet seen, ...
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5answers
3k views

Observer effect, do this mean literally someone or just any interaction with other matter?

I am a layman and was wondering, the quantum observer effect. The regular notion to laymen seems to be literally "if you look at it", but as I am coming to understand the world I live in better I feel ...
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4answers
369 views

Are photons deterministic?

I propose the following scenario: At $t=0$, a photon is emitted from a star. At $t=n$, said photon is received and interpreted by some detector. My question is whether or not it is accurate to say ...
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2answers
136 views

Does performing a measurement on a system change its internal energy?

I'm studying Quantum Mechanics in my spare time from a general point of view (no technical details) so some fundamental question came into my mind: How is it possible to detect a single photon ...
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4answers
1k views

Does the observer or the camera collapse the wave function in the double slit experiment?

Ok so if we setup a camera before the slit we will find a single photon and will follow through accordingly, likewise by having a camera setup after the slit, we can retroactivly collapse the wave ...
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2answers
170 views

About Heisenberg uncertainty principle [duplicate]

What would happen if someone invented a way to measure both position and momentum precisely? If it is impossible why?
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7answers
797 views

Why is Heisenberg's uncertainty principle not an experimental error since it is the error created by photons striking on elementary particles?

Why is Heisenberg's uncertainty principle not an experimental error since it is the error created by photons striking on elementary particles?
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779 views

Can the path of a charged particle under the influence of a magnetic field be considered piecewise linear?

Ordinarily we consider the path of a charged particle under the influence of a magnetic field to be curved. However, in order for the trajectory of the particle to change, it must emit a photon. ...
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2answers
221 views

Does the unpredictability in the “micro world” means that everything is if we can look at it close enough imperfect?

Does the unpredictability in the "micro world" means that everything is if we can look at it close enough imperfect? I mean, there is a saying "You will never stand in the same river again" or ...
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1answer
293 views

Does the measurement on a subatomic particle give its mass value?

No doubt I am wondering about the Young's double slit phenomena. If we observe or measure the properties of a subatomic particle, we are able to know its mass, velocity and it comes to existence. So ...
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2answers
565 views

Is the uncertainty principle just saying something about what an observer can know or is it a fundamental property of nature?

I ask this question because I have read two different quotes on the uncertainty principle that don't seem to match very well. There are similar questions around here but I would like an explanation ...
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What does the quantum state of a system tell us about itself?

In quantum mechanics, quantum state refers to the state of a quantum system. A quantum state is given as a vector in a vector space, called the state vector. The state vector theoretically ...
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1answer
88 views

Is the time of collapse of the wave function empirical?

Is the time of the collapse of the wave function empirical? Suppose there is a very long von Neumann chain of observations of a quantum system. Suppose also practically irreversible decoherence ...
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1answer
263 views

If nothing is “objectively real” prior to “measurement”, what exactly is a “measurement”?

If nothing is "objectively real" prior to "measurement", what exactly is a "measurement"? Is there any "objective" criteria to demarcate a process as being a "measurement" or not? If "measurements" ...
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1answer
166 views

Who are “we”, and what counts as a “question” in consistent histories?

If the preferred basis in quantum mechanics and/or choice of consistent histories in consistent histories is arbitrary, and can only be determined by the "questions we ask", just who exactly is this ...
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1answer
3k views

Application of Heisenberg's uncertainty principle

I've the following application of Heisenberg's uncertainty principle. If a beam of particles in localised in the $x$-direction by a long slit, what is the uncertainty in position? Firstly, I ...
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4answers
601 views

Why, for a spin-½ particle, are the possible outcomes of measuring spin projection along any direction the same?

If one measures the projection of spin of a spin half particle along the $x$ axis one will always get $\pm\tfrac12\hbar$. Measuring it along the $y$ axis one will always get $\pm\tfrac12\hbar$. ...
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3answers
650 views

What is the experiment where subatomic particles appear to foresee the future?

I've seen a documentary, whose name I don't remember but I'm curious because it suggests that subatomic particles are able to "foresee the future". I'll try to describe it here: Some particles are ...
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2answers
607 views

Can we determine whether or not a particle is entangled?

Suppose Shaniqua and Tyrone have four pairs, a, b, c, and d, of entangled particles. They take their particles and go very far apart. If Tyrone can determine whether or not a particle is still ...
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1answer
202 views

In QM, does random data “come from anywhere”? Also, what are the properties of the data?

I have only taken a basic quantum mechanics course (this book, so you know where I'm coming from), but I've been wondering about something. If we set up a quantum system in a known state and take a ...
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0answers
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How to explain Tsirelson's inequality using extended probabilities?

How to explain Tsirelson's inequality using extended probabilities? Some people have tried explaining the Bell inequalities using extended probabilities. For instance, a pair of entangled photons ...
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1answer
111 views

Practical meaning of making a measurement/observation in QM?

When an argument like 'measure the spin along the $x$ axis', 'observe the position of a particle' and so on is made, what is the implied experimental procedure? Since laboratory equipment is ...
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2answers
336 views

Does every measurement correspond to an eigenstate of an observable?

In the postulates of quantum mechanics, physical observables are described by Hermitian matrices on the state space of a system. In another of my questions, the measurements of Rydberg-Ritz spectral ...